As disclosed in documents such as U.S. Pat. No. 7,212,891 (JP 2004-274945A), in an electric vehicle having AC motors mounted therein to serve as an energy source of the vehicle, the AC motors are each capable of serving as a motor for driving wheels of the vehicle as well as a motor driven by an engine to generate power. As the above system, a control apparatus for the electric vehicle includes a voltage boosting converter for raising a voltage supplied by a DC power source, which is implemented by a secondary battery, to a high DC voltage appearing on a power supply line connected to the AC motors through inverters. The inverters are capable of serving as a component for converting the raised DC voltage appearing on the power supply line into an AC voltage for driving one of the AC motors as well as a component for converting the AC voltage into a DC voltage supplied back or restored to the secondary battery through the voltage boosting converter, which lowers the level of the DC voltage.
In the above system, in order to stabilize the voltage appearing on the power supply line, the voltage boosting converter controls the voltage appearing on the power supply line to a target voltage. Further, at the same time, a smoothing capacitor connected to the power supply line smoothes the voltage appearing on the power supply line.
When a relation between electric power driving one of the AC motors and electric power generated by the other AC motor considerably varies due to changes in vehicle operating state or another reason, however, a voltage variation caused by a change in such relation as a voltage variation of the power supply line cannot be absorbed by the voltage boosting converter and/or the smoothing capacitor. Thus, the voltage appearing on the power supply line becomes excessively high. As a result, it is likely that electronic equipment connected to the power supply line is damaged. In order to cope with this problem, there is provided a method for enhancing the effect of stabilizing the voltage appearing on the power supply line by using an improved voltage boosting converter with better performance and a smoothing capacitor with a larger capacitance. By adoption of this method, however, the voltage boosting converter with better performance and the smoothing capacitor with a larger capacitance will inevitably raise the cost of the control apparatus for an electric vehicle. Thus, demands for a system having a small size and a low cost cannot be met. The above relation between the power driving one of the AC motors and the power generated by the other AC motor is also referred to as a balance of power between the power driving one of the AC motors and the power generated by the other AC motor.
It is proposed for controlling the inverter to make a sum of energies (or balance of electric power) of the two AC motors equal to 0 at the time the connection between the DC power source and the voltage boosting converter is cut off by using a relay in the event of a failure occurring in the DC power source. However, this method is provided as a countermeasure to a failure occurring in the DC power source and is capable of enhancing the effect of stabilizing the voltage appearing on the power supply line in a normal state of the power supply. In addition, even if an attempt is made to control the inverter to make the sum of energies (or the balance of power) of the two AC motors equal to 0 in a normal state, it is difficult to control the inverter to make the sum of energies (or the balance of power) of the two AC motors equal to 0 in the following cases.
In the first place, one of the AC motors is linked to a driving shaft of the electric vehicle and the other AC motor is linked to an output shaft of the internal combustion engine, that is, the two AC motors are linked to members having different operations. In the second place, the effect of a processing delay of the control executed on the inverter becomes larger, for example, during a transient condition in which the operating state of the electric vehicle changes. The AC motor linked to the internal combustion engine is not capable of obviating power variations caused by changes of a torque generated by the internal combustion engine. This fact makes it even more difficult to control the inverter to make the sum of energies of the two AC motors equal to 0.